פירוש על עבודה זרה 38:13

Rabbah b. Ulla explains R. Yirmiyah’s reasoning. The prohibition of the benefit derived from the idol begins only when the last stroke in making the idol is completed, for only then is it an idol. The benefit derived from that last stroke would be prohibited, but the value of this last stroke is less than a perutah—it is negligible. The amount he was paid for the work done prior to this point is considered to have already been paid before the work was completed. This means that these wages, which are essentially all of the wages, are permitted.
There is a dispute in Tractate Kiddushin about how we conceive of wages paid to a person being paid not by the hour but by the project. Are all of the wages thought of as being paid at the completion of the project or piece by piece as the project is created? Rabbah b. Ulla says that R. Yirmiyah holds by the latter position.