What legal agreement is needed for investing in a business?

I need help drafting a legal agreement for investing or providing loan funds to an existing sole proprietorship firm (with GST registration) with fixed returns and no risk. Query: • The repayment period should be 24 months. • A fixed return of 5% on the invested amount must be paid, regardless of the business's profit or loss. • What legal structure should be used for such an agreement (e.g., Promissory Note, Business Loan Agreement, Friendly Loan Agreement, MoU, or Hundi)? • Since individuals cannot legally charge interest without a money lending license, can this return be termed as royalty, profit share, or some other legal term? • The investment should be risk-free, considering the business was profitable at the time of investment and had assets.