A week from today — that is, on July 21st — I will repay you.
Literal
Next-week [possessive-の] today, that-is July 21 [on-に] you [to-に] repay-humbly-do.
すなわち ('that is / namely / in other words') is a formal written connective used to clarify or rephrase. 返済 ('repayment') is a formal suru-verb noun. いたします (humble する) gives the sentence business formality. A classic financial commitment phrasing.