Praise be to Allah the Lord of the Worlds. May His peace and blessings be upon our Prophet Mohammad and upon all his family and companions.
From the perspective of Sharia, debt is the right of creditor, so he/she may forgive the debtor or relinquish part of the debt. The proof of this is that Allah the Almighty says {What means}: "If the debtor is in a difficulty, grant him time Till it is easy for him to repay. But if ye remit it by way of charity, that is best for you if ye only knew." {Al-Baqara, 280}.
The funding due on the Murabaha contract is considered debt and the investment fund is considered creditor, so the latter is allowed to reduce the value of the profit due on old funding and determine the amount of the reduction considering that the value of the funding due on clients is the right of the fund.
In conclusion, it is permissible for the fund to reduce the Murabaha ratio for the old funding. However, if savers or investors possess the funds of the fund then their consent must be sought prior to issuing the decision of reducing the ratio of the old Murabaha transactions. And Allah the Almighty knows best.
*Murabaḥah, murabaḥa, or murâbaḥah was originally a term of fiqh for a sales contract where the buyer and seller agree on the markup or "cost-plus" price for the item being sold.