פירוש על קידושין 96:6

All tannaim hold that one cannot betroth by simply forgiving a debt. That is why all agree that with regard to back wages, one cannot use them for betrothal. But the first tanna holds that one may betroth with future wages. This tanna would hold that as soon he finishes the work, he becomes entitled to the wages and by forgiving them he betroths her. There is no debt incurred. R. Natan would hold that the wages are incurred incrementally and therefore this is betrothal with a debt.

R. Natan would hold that even if he adds something of his own and gives it to her, when a man betroths by forgiving a debt and with a perutah, her mind is on the debt. Therefore, adding something of his own does not help make this a valid form of kiddushin.