Hi, thank you for your question. Just a bit more information required to fully assist you:
-How much is the property worth and what is the outstanding mortgage?
-Do you have any formal agreements or deeds of trust in place - if so what are the terms?
-Is the property held as joint tenants or tenants in common on the title?
In relation to joint tenants or tenants in common on title you will need to obtain the title deed which can be done here: https://www.gov.uk/search-property-information-land-registry
If the following wording is on the title, then you are tenants in common, if it is not you are joint tenants: "No disposition by a sole proprietor of the registered estate (except a trust corporation) under which capital money arises is to be registered unless authorised by an order of the court".
In relation to the transfer, what has he offered to buy your share and are you in agreement with his offer?
Thanks for confirming. Firstly, your legal interest in the property will depend on what share and how the title is held on the title deed. If it is as joint tenants or in equal shares you are legally entitled to 50% of the proceeds of sale.
Furthermore, if he is seeking to buy-out your share then you would be best placed to request for there to be a market appraisal by an estate agent to ascertain what the current value of the property is and base your negotiations around that. On your figure of £145,000, you would be entitled to approximately £6-7,000 which takes into account any potential costs of sale at 2.5%.