ANSWER:

This is not permissible at all as these are two different obligations on you, you must give them separately and must not combine. 

I made a mannat (منت) to give 10k money when my dua gets accepted. I am now zakaat eligible and the zakaat I have to give this month is around 10k as well. Can I give zakaat and combine the intention of mannat too? (I am giving money before dua is manifested because when I have to give away 10k zakaat, why not to add intention of mannat too) Please guide if this is allowed it not