Lewis & Short

prō-mūtŭus, a, um, adj.,

  1. I. paid over beforehand, advanced, loaned: publicanis (imperabatur) insequentis anni vectigal promutuum, was demanded in advance as a loan, Caes. B. C. 3, 32 fin. (for which before: insequentis anni mutua pecunia, id. ib. 3, 31, 2): vectura, prepaid, Dig. 19, 2, 15.
  2. II. Subst.: prōmūtŭum, i, n., an advance, loan, Dig. 40, 7, 40, § 5.