Is it permitted for them to make this arrangement up front (פסיקה)?
The Rambam seems to say no. The Rambam is only מתיר a ת"ח to return more (without any pre-arangement at the time of the loan). The Gra learns that the Rambam is מדקדק from the fact that the Gemara said ללות and not להלוות. The obvious question is the המשך הגמרא. The Gemara brings the story of Shmuel where he seems to be פוםק beforehand that he will borrow 100 and return 120. The Gra explains that the Rambam understood that Shmuel was not being פוסק but rather was just raising the possibility that he might return more because he will not be מקפיד to count. The Shulchan Aruch paskens like the Rambama however, the Rama paskens that it is even מותר to be פוסק beforehand.