What was given to you, is yours already. You wanna keep/return/throw away - that is your call.
BUT if he gave you some family heirloom, or childhood stuff like photos, or anything ,of sentimental value to him and his family (awards, trophies, markers of achievements) -- darling, soli mo yan. Oo, kahit na tempting to keep that 5k ring - if galing sa mommy or grannie nya - return it.
But if he bought you that 5k ring, and asks for it back - lol, remove the diamond and keep it - Send him back the ring/setting. Lol. This happened in real life. (Engagement called off coz girl caught husband to be - may GF/mistress pa lang binahayan and goes there regularly. Then he had the kapalmukz to write her asking for the engagement ring back. So she sent him the ring, without the 5k stone. Lol)