The allowed maximum number of decimals for the Invoice line allowance base amount (BT-137) is 2.
//cac:InvoiceLine/cac:AllowanceCharge[cbc:ChargeIndicator = false()] | //cac:CreditNoteLine/cac:AllowanceCharge[cbc:ChargeIndicator = false()]
string-length(substring-after(cbc:BaseAmount,'.'))<=2