Responsa על בבא מציעא 187:10

Q. B lent money to A. The latter promised to give to the former half of the profits he would derive from all his business transactions, including those transactions in which money borrowed from a third party would be employed. Is this promise binding on A?
A. If the promise was accompanied by a kinyan in the presence of witnesses, it is binding; otherwise it is a mere asmakhta (exaggerated promise) and is not binding.
SOURCES: Pr. 676.