Spanish to English Bus/Financial Accounting

The whole sentence goes: "La Sociedad ha realizado los pagos acordados según el calendario establecido, habiendo registrado el beneficio correspondiente a la quita por la parte proporcional al importe amortizado, así como por la parte garantizada mediante aval bancario constituido."
I believe "la quita" is write-off but what is throwing me a bit is the use of "por". Has the corresponding profit been written off by the proportional part of the amortised amount??? Or is the profit being used to write off the amortised amount? Neither seems to make much sense to me.