As for paying Zakah, shall I pay Zakah on the amount by which I bought the land or the amount by which I will sell it? Please clarify the matter for me, may Allah reward you!.
It seems from your question that you want to build on this land that you bought and make the ground floor of it into a mosque, and that you made this vow out of gratitude to Allaah for having enabled you to buy this land, but now you cannot build on it and you are asking whether it is permissible to sell it.
No, it is not permissible for you to sell it, because you vowed to make the ground floor of it into a mosque, and to make the upper floors into accommodation. So long as you cannot afford to build it at present, then you must wait until Allaah enables you to build on it and fulfil your vow, because you did not despair of being able to afford it and fulfil this vow until now.
You do not have to pay zakaah during this period, because you did not intend to sell it, rather you intended to build on it.