Commentary for Kiddushin 125:11

Resh Lakish could resolve those baraitot by connecting them to a tannaitic dispute as to how to reckon wages. When one performs work for another, does that person become liable to pay the worker all at one time or incrementally? If incrementally, then at no point is he giving her the value of a perutah, because we could divide the moments up into units worth less than a perutah. This is the opinion of our mishnah—wages cannot be used for betrothal because there is no point at which they are worth a perutah. The other baraitot hold that the obligation is incurred only at the end of the labor, when they are worth a perutah.