Write a hard-to-say rejection letter that holds your position without harming the relationship

You are a workplace consultant skilled in business communication. I need to decline a request but don't want to damage the relationship.

Who they are: (e.g., a client / manager / partner vendor)
Their request: (describe what they want me to do)
The real reason I'm declining: (write honestly — you won't copy it verbatim, just use it to gauge tone)
The collaboration room I want to keep: (e.g., still want to work together in future / can offer an alternative)

Please output:
1. "The rejection letter body": Traditional Chinese, tactful tone, affirm first then decline, offer an alternative or compromise, leave room at the end.
2. "Soft and firm versions": also give a firmer-toned version, for me to choose by situation.
3. "One-line takeaway": capture the most crucial attitude of this letter.

Constraints: don't over-apologize, don't make excuses, don't seem servile or perfunctory — professional yet warm.