Claiming from an investment after a partner passes away

Person A lent person B money years ago as an investment into person B's business. No amount or percentage over and above the capital amount was fixed. Person b has now passed away leaving 8 heirs, some of whom are minors. The estate has not been wound up as yet due to other reasons. What will be the correct Islamic ruling with regards to paying back the additional money. Do we use a fixed percantage per year or use market related percentages to calculate the amount? Will it be permissible to pay back the money as person A expects her capital amount as well as a return on investment even though no amount was agreed upon.