Live foal guarantee is a common provision in horse breeding contracts.
But many stallion owners will offer a reduced stud fee in such cases, for goodwill reasons.
Typically, after three unsuccessful tries, the stallion owner will either require substitution of another mare or will return the stud fee.
This is to ensure that the mare is healthy, able to carry a foal, and to check for genetic incompatibilities.
There is no point to re-breeding in such cases, so the stallion owner will allow substitution of another mare or will return the stud fee.