Does Paul lie or not?
Roman 3:7 But if through my lie God’s truth abounds to his glory, why am I still being condemned as a sinner?
Galatians 1:20 I assure you before God that what I am writing you is no lie.
In Romans 3:7, Paul makes a list of comparisons to show up the illogicality of him being accused of being unrighteous. In verse 8 he points out that some people are slanderously accusing him as urging evil to be done, in order that God's goodness shine forth. He was dealing with what has come to be labelled "antinomianism". That is, "against law". Lawlessness. Violating God's good laws due to a warped doctrine of having been totally forgiven by God.
He starts by saying that if some people do not believe, that will not make the faith of God without effect (verse 3). He continues with saying that just because men are liars, that does not affect the truth of God being true (verse 4). Thirdly, he argues that although Christians are unrighteous, that does not affect God being totally righteous (verse 5).
The crunch point comes next, which relates to the question. If Paul is compared with God, and judged to be a liar while God remains true, his being judged as a sinner serves to show the righteous judgment of God. For "There is none righteous, no, not one... there is none that doeth good, no, not one. Their throat is an open sepulchre; with their tongues they have used deceit; the poison of asps is under their lips: Whose mouth is full of cursing and bitterness..." and so forth (verses 10 to 18).
The people he was writing to knew the ten commandments, which includes "Thou shalt not lie." But where is the person who has never lied, who has never been deceitful in some respect? So, with confidence, Paul goes on to say that the law was written to those
"who are under the law: that every mouth may be stopped, and all the world may become guilty before God. Therefore by the deeds of the law there shall no flesh be justified in his sight: for by the law is the knowledge of sin. But now the righteousness of God without the law is manifested, being witnessed by the law and the prophets; Even the righteousness of God which is by faith of Jesus Christ unto all and upon all them that believe: for there is no difference: For all have sinned and come short of the glory of God, being justified freely by his grace through the redemption that is in Christ Jesus." Romans 2:19-24 K.J.V.
Paul was building up the case for men being liars (including himself) but God being found true. It would not matter if every man be found a liar. God remains true. And when he judges every man, his righteous judgment will be undeniable. Every mouth will be shut before him. In other words, nobody who tries to justify themselves before God will have a leg to stand on. So, Paul does not even try to justify himself. Nor should any Christian.
Does any of that contradict what Paul then wrote in Galatians 2:20? Well, he had warned them not to listen to perverted gospel messages, then gave them a summary of his conversion to Christ after having wickedly persecuted Christians. His conversion testimony is what he speaks of in verse 20:
"Now the things which I write unto you, behold, before God, I lie not."
Then he continues with the account of his life, through the next 14 years till the present. His readers knew he was giving a truthful account of all those things, and that his presentation of the gospel message was truthful. They could depend on the gospel message he preached, unlike those deceivers who taught a false gospel.
The answer is, "There is no conflict. There is no contradiction. Paul did not lie, either in the book of Romans, or in his letter to the Christians in Galatia."
Romans 3:7 needs to be understood in the context of an earlier verse in the same chapter:
4 God must be true, though every human being is a liar.
Thus, Paul admits that he - like everyone else - sometimes lies. This does not mean he always lies. In Galatians, he makes a specific truth claim. He refers to his account of his early history as a Christian, especially that he received his version of the gospel through revelation, and that it was later approved by the pillars of the Jerusalem church:
that the gospel preached by me is not of human origin. 12 For I did not receive it from a human being, nor was I taught it, but it came through a revelation of Jesus Christ. 13 For you heard of my former way of life in Judaism, how I persecuted the church of God beyond measure and tried to destroy it, 14 and progressed in Judaism beyond many of my contemporaries among my race, since I was even more a zealot for my ancestral traditions. 15 But when [God], who from my mother’s womb had set me apart and called me through his grace, was pleased 16 to reveal his Son to me, so that I might proclaim him to the Gentiles, I did not immediately consult flesh and blood, 17 nor did I go up to Jerusalem to those who were apostles before me; rather, I went into Arabia and then returned to Damascus.> 18 Then after three years I went up to Jerusalem to confer with Cephas and remained with him for fifteen days. 19 But I did not see any other of the apostles, only James the brother of the Lord.
Conclusion: In Romans, Paul admits that he is a liar, as are all human beings. (Ps. 116:11) However, with regard to his personal history in Galatians 1, Paul affirms that he is telling the truth