Fear of breaking an oath

Q: Someone lost money and I found it. They said that I should keep it. I said "wallahi I'm not gonna take it"Their intention was to keep it as a gift. After I gave it to them they gave it to me to put in the sadaqah tin. Because I took it in my hand do I have to give kafaraah? My intention when I said "wallahi I wont take it" I meant as a gift. 

Forgetting the number of rakaats in salaah

Q: During a four rakat prayer, if one has forgotten whether they are in rakat two or three, and they decide two is more likely so they sit and read attahiyyat, do they also have to read tahiyyat after the next rakat, which they assume is the third one?

If one is not sure whether they are in rakat one or two, and they decide two is more likely so they sit and read attahiyyat, do they also have to read attahiyyat after the next rakat?