To put it bluntly, tell it like it is.
You are not in a position to confirm your full time return and whilst you would like to make a start back you know you are not fit enough to resume full time duties and do not wish to upset the temporary manager. You might also be advised medically not to return so soon.
You employers clearly have to renew the temporary manager's contract.
Put all of this to your employers. It shows a willingness to work but you need to bear in mind they have a duty of care to you and if they have any sense will want you to get the all clear medically before returning. My feeling is that they should and will refuse your offer unless you get medical clearance, however well intentioned it is.
I hope this assists. If there are any further points kindly reply I will be happy to respond.
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