I am not sure if this is the case for this Question but often the question, "When was Jesus rich?" tacitly interprets the phrase "although he was rich" (2 Cor 8:9) through these verses at Philippians ...
[Jesus] though he existed in the form of God
did not regard equality with God
as something to be grasped,
but emptied himself
by taking on the form of a slave,
by looking like other men,
and by sharing in human nature. (Phil 2:6-7 NET)
... and tacitly assumes that Jesus Christ was not rich in any sense, while he walked on earth, and therefore he must have been rich in a "previous life", in his "pre-incarnated state".
It is necessary to understand 2 Cor 8:9 in the context of the whole pericope 2 Cor 8:1-15.
That context is about the grace of giving so that others might be rich. Paul exhorts Christians to graciously give out of what they have so that others might also have. And Paul turns to Jesus as an example for them to follow. Paul instructs them to give out of their abundance so that other saints might also have abundance.
Edited to add (June 3, 2021, 23:15 CET)
The parellel between 2 Cor 8:1-15 and Philippians 2:1-11 is even stronger, when we meditate on this verse:
“Have this attitude in/among yourselves which also [was] in Christ Jesus,” (Philippians 2:5)
Edited to add (June 4, 2021, 19:15 CET)
It has been pointed out to me, in the Comments to my Answer, that the equation ...
rich = Son of God
... may suggest to some that "Jesus pre-existed his incarnation". This is certainly not what I think (in fact I consider the thought a disaster).
Jesus is the Son of God because he is the incarnation of God's logos (John 1:14) and this very incarnation took place thanks to the operation of God's hagion pneuma in the Virgin Mary (Luke 1:35).
P.S. To pre-empt any further misunderstanding, God's logos and pneuma are not "persons", but essential attributes of God.