There is nothing to stop you and your friend having any agreement you like. It would be better done by deed which needs to be witnessed because that makes it more official.
Even if the friend will not agree and whoever has the property at the moment and who you are looking after decides to cut you out of the will in favour of your friend, and you spent money and time looking after the personal, and you did so on the basis that they promised they would leave you the property, they would be denied honouring the promise under a legal doctrine called Promissory Estoppel.
You need to have had a promise made to you and then relied on that promise to your detriment which, in this case, you would have done by spending time and money.
So you see, there are actually 2 ways of dealing with this.
It would be essential that your friend takes legal advice from an independent lawyer on the effect of entering into the agreement for it to be enforceable.
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