Nothing in the commandments of the Israelite Scriptures has been loosened by Jesus unless He makes a declaration against a commandment or concerns a past generation only. Deuteronomy 4:1-2, 5:31-33.

The dismissed woman is not allowed to go back to the former man if the woman had been with another man, even after the other man died (Deuteronomy 24:1-4).

Deuteronomy 24:1-4 ASV
(1)  When a man taketh a wife, and marrieth her, then it shall be, if she find no favor in his eyes, because he hath found some unseemly thing in her, that he shall write her a bill of divorcement, and give it in her hand, and send her out of his house.
(2)  And when she is departed out of his house, she may go and be another man's wife.
(3)  And if the latter husband hate her, and write her a bill of divorcement, and give it in her hand, and send her out of his house; or if the latter husband die, who took her to be his wife;
(4)  her former husband, who sent her away, may not take her again to be his wife, after that she is defiled; for that is abomination before Jehovah: and thou shalt not cause the land to sin, which Jehovah thy God giveth thee for an inheritance.

Matthew 5:17-18 ASV
(17)  Think not that I came to destroy the law or the prophets: I came not to destroy, but to fulfil.
(18)  For verily I say unto you, Till heaven and earth pass away, one jot or one tittle shall in no wise pass away from the law, till all things be accomplished.

Mark 10:9 EMTV

(9) Therefore what God has united together, let not man separate."

Jesus did not abolish the commandments of the Holy Scriptures of the Hebrews (Matthew 5:17-18) unless He does so explicitly like in forbidding the breakup of a man and a woman (Mark 10:9) after they cleave to each other.