Is it permissible for me to take usurious interest from my friends whom I helped in a contracting project, knowing that they only returned the principal to me?

If you handed over the money on the basis that you are a partner in profit and loss, without stipulating a guarantee of the principal, then they are obliged to give you your share of the profit after deducting the wages of those who executed the project. The net profit is calculated, and you take a percentage corresponding to your contribution.

However, if you stipulated interest along with a guarantee of the principal, then this is an usurious loan, and it is not permissible for you to take interest. Only your original principal should be returned to you, based on the Almighty's saying: "And if you repent, then you shall have your principal; neither oppress, nor be oppressed."