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But if this is your biggest contract by far and it is a spare-time kind of thing, just should take what you can get. The company is not going to jeopardize its negotiations about a purchase with what it might consider to be a "poison pill". There are always lots of issues on the table in a merger/takeover negotiation. The terms of a recently signed contract does not need to be another one of those issues on the table.
If the business is a legal entity in its own right, then you have potentially less to worry about, as the contract will have been taken on by the company and a change of ownership should be irrelevant to them honouring their obligations under it.
With a sole propriteor and a partnership - depending on where you are in the world - you have to get legal advice and get a suitable contract drawn up, as it may be difficult to enforce if you need to.
That being said, I would suggest that you talk to a lawyer who specialises in company law anyway and get them to draft a contract for you, which has a clause that deals specifically with this issue.
PericlesStrategy - The company is a Plc. Your point about the change of ownership but is very interesting and something I hadn't realised.