Yes, bot can place bets below Betfair minimum size.
But you should also place regular bets with sizes above Betfair minimum size or that satisfy Betfair "Minimum Bet Payout" rule.
Beside minimum bet size Betfair also has rule on "Minimum Bet Payout" and this means that all bets that have liability higher than "Minimum Bet Payout" are considered valid bets and not below minimum size. This allows you to bet at higher odds with very small amounts of money e.g. bet £1 @ 10, or 10p @ 100 or 1p @ 1000 are all valid as they all target payout of £10 or more. By Betfair rules, "Minimum Bet Payout" functionality is not available for .it, .es and .dk jurisdictions.
To place bet below Betfair minimum size you must have at least available amount to bet plus the amount that you wish to place. If this amount is not available to bet then process of placing bet below minimum size will not work and bet might not be placed.
Betfair minimum bet size depends on currency and type of bet. On image below you can see detailed minimum bet sizes table for each currency and bet type that is provided by Betfair.
19 June 2020 Changes related to INVALID_PROFIT_RATIO error
On 19th June 2020 Betfair added new functionality to reject any "unfair bets" and error INVALID_PROFIT_RATIO is returned. This effects very small number of bets that are below minimum bet size and take advantage when rounding numbers to two decimal places.
Any "unfair bets" below minimum size will be rejected by Betfair API automatically. Here are examples of unfair bets that bot is unable to place and which would be rejected by Betfair:
Lay £0.02 at odds of 1.65 would create liability of £0.013 but it would be rounded to £0.01, creates less liability than it ought to.
Back £0.01 at odds of 1.7 would create profit of £0.007 but it would be rounded to £0.01, winning you more than it ought to.
Any bet that results in returns 20% less or 25% more than it ‘ought’ to will be rejected by Betfair automatically.
Software is updated to follow those rules and it will allow you to place bets that are below minimum size and satisfy those rules. Here are examples of bets that can be placed below minimum size:
Lay £0.02 at odds of 1.44 would create liability of £0.088 and it would be rounded to £0.01.
Back £0.02 at odds of 1.55 would create profit of £0.011 but it would be rounded to £0.01.
When placing Lay bets below minimum size software is no longer able to use temporary Lay bet at odds of 1.0. Instead it needs to use temporary bet at higher odds so that bet satisfies minimum payout of £0.008. This can result in placing temporary "£2 + desired bet size" bet at higher Lay price. Here is an example for process of placing Lay £0.02 bet at odds of 1.6:
1. place Lay £2.02 bet at odds of 1.41
2. update bet size to £0.02
3. update bet price to 1.6
If we used lower price than 1.41 then this would not work as our £0.02 bet would than create smaller liability than £0.008 in which case Betfair would reject whole bet.
This also means that bets £0.01 at odds lower than 1.8 are no longer possible, as they are considered "unfair bets".
