Here is our base guarantee clause. You should use something similar to this.

The Recruiter provides a six (6) month guarantee on each Candidate placed with the Client ("Guarantee Period"). If the Candidate's employment ends within this period for reasons other than redundancy, restructuring, or misconduct by the Client, the Recruiter may either:

• Present a suitable replacement at no extra fee; or
• Refund the original placement fee.

The Guarantee applies only if:

• The Client notifies the Recruiter in writing within five (5) business days of termination and provides relevant details;
• All placement fees have been paid in full and on time;
• The Candidate's role, pay, or location have not been materially changed;
• Termination is not due to misconduct or unlawful behaviour by the Client.

The guarantee is limited to one (1) replacement or one (1) refund per placement. If a replacement is provided, no further guarantee applies.