AI would always accept if you send some money together with the city.. In my tests $2,500 - $3,000 is enough. If you give less, it would be suspicious
I don't think force diplomacy is good for this, since a razed city is almost useless and the faction taking it would have to waste a lot of $$$ for garrison unkeep. I had used the trick to successfully destroy AI economics
Also remember to drain out all mercenaries in that region... It should be able to prevent the city from receiving good garrion troops.