Out of curiosity, does this means she pays on the tenth day of the month, or that she pays a tenth of [whatever she owes] every month? Or is it ambiguous?
And if it's the second option (a tenth of some amount), would "tithe" be an acceptable translation?
It means that she pays on the tenth day of the month and it is not ambiguous.
There used to be a tax called 'tiende' under Sigurd Jorsalfare's (Sigurd the Crusader) rule back in 1103-1130, and you paid 1/10th of your income to the church.