In order to place back bets with stakes less than the Betfair minimum (£2 in the UK), TheBetEngine has to undertake a set series of steps. Let's illustrate it through an example:
I want to place a bet for 50p at odds of 3.2
In this case, TheBetEngine will have to do the following:
1. Place a bet for £2 at odds of 1000 (this is not normally matched)
2. Adjust the stake to be £2.50 - the odds remain at 1000 (Betfair internally splits this into two bets - one at £2, one at 50p)
3. Cancel the £2 bet
4. Adjust the odds to be 3.2
Now Betfair always matches a back bet at the best odds available, so if the market is only offering odds of 1000 at the point that the bet in the first step is processed, then Betfair will match the bet, leaving you with a matched bet of £2 at odds of 1000.
Unfortunately once a bet is matched it cannot be altered.
Assuming normal operation, this actually happens very rarely so in 99.99999% of the cases, the initial bet will go unmatched and the rest of the sequence can be undertaken.