I think using a Shared Tab would be a great way of sorting this:
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Create a Shared Tab between you and your friend.
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Add a custom bill for what you owe them (£5,000) and allocate the entire amount of the custom bill to yourself.
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Every time you pay your friend you can add it to the Shared Tab as a custom bill.
A few improvements to Shared Tabs which would help your suggested situation would be:
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With the ability to partially settle a Shared Tab you could pay an amount directly into the Shared Tab whenever you want. You can vote for the ability to partially settle a Shared Tab here.
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With the ability to schedule recurring payments into a Shared Tab you could set up a recurring payment for £200 into the Shared Tab every month until the Shared Tab is paid off. You can vote for the ability to schedule recurring payments into a Shared Tab here.