Praise be to Allaah.
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.
Majmoo’ Fataawa al-Shaykh Saalih al-Fawzaan (1/104).