The LXX translators didn't have the vowel points of the MT, that weren't added until centuries later. The difference is the vowel points. Also note that חָמָֽס is the common word for violence in Hebrew, but can also mean wrong. In Arabic it means zeal and is the name of a political group in the Middle East.
The vowel points in MT שֶׁ֥בֶת (based on Codex Leningradensis)
†I. שֶׁ֫בֶת S7674, 7675 TWOT2323a, 922a GK8699, 8700 n.f. seat, dwelling, place (properly Inf. Qal from ישׁב)
Brown, F., Driver, S. R., & Briggs, C. A. (1977). Enhanced Brown-Driver-Briggs Hebrew and English Lexicon (p. 443). Oxford: Clarendon Press.
The vowel points for Sabbath - שַׁבָּת
שַׁבָּת S7676 TWOT2323b GK8701111 n.f. Ex 31:14 m. Is 56:2, 6; 66:23, (under infl. of יוֹם in freq. יוֹם הַשּׁ׳, Albr xvi (1896), 47) Sabbath (= ת + שׁבת; NH שַׁבָּת
Brown, F., Driver, S. R., & Briggs, C. A. (1977). Enhanced Brown-Driver-Briggs Hebrew and English Lexicon (p. 992). Oxford: Clarendon Press.
†חָמָס S2555 TWOT678a GK2805 n.m. ψ 7:17 violence, wrong
Brown, F., Driver, S. R., & Briggs, C. A. (1977). Enhanced Brown-Driver-Briggs Hebrew and English Lexicon (p. 329). Oxford: Clarendon Press.
Moving to Answer
The verbs of the verse make it antithetic parallelism, which makes sense with the MT vowel points.
False sabbaths σαββάτων ψευδῶν doesn't make sense neither as Hebrew poetry or the context of Amos.
וַתַּגִּישׁ֖וּן is is hiphil, and means "cause to approach." Thus, meanings as bring.
Refining the Answer
The translations from KJV to present accept the MT vowel points, but have a wide variety of translating the verse.
Senses of usage in MT:
Yet you ward off [the thought of] a day of woe
And convene a session of lawlessness. (JPS)
Do you put off the day of calamity,
And would you bring near the seat of violence? (NAS)
Woe to you who put far off the day of doom,
Who cause the seat of violence to come near; (KJV)
You put off the evil day
and bring near a reign of terror. (NIV)
O you who put far away the day of disaster
and bring near the seat of violence? (ESV)
You dismiss any thought of the evil day
and bring in a reign of violence. (HCSV)
qui separati estis in diem malum et adpropinquatis solio [throne, seat] iniquitatis
(Biblia Sacra Vulgata)
Note: the Vulgate predated the masorah. Thus Jerome took the the vowels the same as the masorah.
Ver. 3 begins the further explanation of the careless security charged in ver. 1. Regarding the evil day, i. e., day of judgment as far off, they cause violence to erect its throne nearer and nearer among them. [Pusey follows Jerome, Grotius, Newcome, and others in referring the throne of violence to the rule of Assyria, which the people brought nearer to them while they were thinking to put it far off. But the former reference is much more natural.]
Lange, J. P., Schaff, P., Schmoller, O., & Chambers, T. W. (2008). A commentary on the Holy Scriptures: Amos (p. 41). Bellingham, WA: Logos Bible Software.
He means, that they drew nigh the throne of iniquity, when the judges strengthened themselves in their tyranny, and took the liberty to steal, to rob, to plunder, to oppress. When therefore they thus hardened themselves in all kinds of licentiousness, they then drew nigh the throne of iniquity. And they put away the evil day, because they were touched by no alarm; for when the Prophets denounced God’s vengeance, they regarded it as a fable.
Calvin, J., & Owen, J. (2010). Commentaries on the Twelve Minor Prophets (Vol. 2, p. 306). Bellingham, WA: Logos Bible Software.
While the translations vary, the MT vowel points make the most sense. Note: The vowel points didn't exist for the Dead Sea scrolls and the LXX translation doesn't indicate a difference in the consonants. The textual apparatus doesn't show variation in the consonants.
Masorete (also Massorete) -- Any of the Jewish scholars of the 6th–10th centuries AD who contributed to the establishment of a recognized text of the Hebrew Bible, and to the compilation of the Masorah [included written vowel points].
‘This was the work of the Massoretes and the resulting text only began to be produced from the sixth century at the earliest.’
As the literacy of society increased, the Masoretes worried that students might stop memorizing the vowels for the text of the Tanakh. So, they developed a method of writing down the vowels to preserve them.
Some example images of Dead Sea scrolls (only consonants, no written vowels):