Calling on Allah to bear witness that one will do or will not do something comes under the ruling on oaths, if it was intended as such. So if you intended to swear an oath when you said that, then it is an oath, and you may accept the money and offer expiation for breaking your oath. See the long answer for more details.

The words {from the average of that which you feed your [own] families} [al-Maa’idah 5:89], regarding the expiation for breaking an oath, do not mean that the one who wants to offer this expiation must be the one who feeds his family. Rather this is by way of explaining that the food should be the average of the type of food that he or his family eat.