Is it permissible to take the principal amount and an equivalent amount as a loan to compensate for expected profits from an investment project, knowing that the currency's value has deteriorated?

Selling your share to your partner for the amount of the capital is permissible. If your partner wishes to lend you a goodly loan (qard hasan) as a donation from him, and separate from the sale, there is no objection to that. However, if the loan is conditioned upon the sale, then it is not permissible, because it combines a loan and a sale, which is forbidden by Islamic law and leads to usury (riba). Therefore, it is necessary to ascertain the reality of the loan: Is it a pure donation not conditioned upon the sale, or is it conditioned upon it? If it is the former, it is permissible; if it is the latter, it is forbidden.