How to get the customer Invoice trxn id in the Java validation at receipt creation level

I'm creating a custom validation to be used at Order level in the receipt creation program. I'm trying to find the Customer Invoice details. In the java method used for validation, these are the available input parameters as mentioned in the implementation guide. I'm not able to derive the Customer trxn id using these parameters.

This is the Java validation method with the signature :
ValidationSetResult validate(Integer paramInteger, Payee paramPayee, PmtInstr paramPmtInstr, Tangible paramTangible, Transaction paramTransaction).

How do we get the Customer Trxn Id using this ?

For example, to get the PayerInstrumentId , we use
BankAccount localBankAccount = (BankAccount)paramPmtInstr;
localBankAccount.getInstrAssignmentId();