Is it obligatory to pay zakat again if, when it was first paid, the intention was a combination of zakat and sadaqah, knowing that the wealth only reached the nisab last year? And is the questioner sinful if he stops giving the sadaqat that he committed himself to, for the sake of marriage? And when and how is zakat to be paid if it becomes obligatory again?

Zakat is not obligatory unless the wealth reaches the nisab (threshold), and it is a condition for zakat that there be an intention of pure obligatoriness. If you combine zakat and charity with the intention of the obligatory zakat and whatever exceeds that as charity, there is no harm. However, if the intention was singular without purifying it for the obligation, then you must re-issue the zakat with the intention of zakat only. Zakat is to be given from the currency itself, or by its value at the time of disbursement if it is in another currency. If you make a vow internally without uttering it, nothing is binding upon you. However, if you utter it, then it is an oath and a vow that must be fulfilled. It is permissible to interrupt it when necessary and then return to fulfilling it after the excuse is removed.