Betfair sometimes offer net exchange losses refunds to some customers. You’ll be notified by email if you’ve received one of these offers. They can be for different amounts, but the most common is 100% losses refunded up to £50. It’s important that you place only one exchange bet during the qualifying period, so you can make sure your net losses are exactly right if your back bet loses.

Betfair Refund

The simplest way to take advantage of this offer is simply to treat it as an SNR free bet on the Betfair exchange. You can use the OddsMatcher to find the best current match by selecting Betfair Exchange as the Bookie, Free Bet (SNR) as the Type, and entering the refund you’ve been offered as the stake. 
You need to place your back bet on the Betfair exchange and lay it on a different exchange. You can adjust the OddsMatcher settings (click the cog in the top-left corner) to select the exchange for the lay (e.g. smarkets).
Betfair Refund
When you place your back bet on the Betfair exchange, it will use your own cash. However, this will be refunded if the bet loses, making this effectively the same as a standard SNR bet.