Best method to find variances between checkbook deposits and GL deposit transactions

One idea to consider is a recursive lambda function that processes one check at a time, assigning the check number to as many GL entries as necessary before processing the next check. 

The function RECONCILE is defined like this, with comments for clarity:

RECONCILE = LAMBDA(checks, ledger_entries,

    LET(
        // Add an empty column for the ledger check numbers if necessary
        ledger_entries,IF(COLUMNS(ledger_entries)=1,HSTACK(ledger_entries,EXPAND(0,ROWS(ledger_entries),1,0)),ledger_entries),
        // get the column with the ledger check numbers
        ledger_checks, TAKE(ledger_entries,,-1),
        // get the ledger amounts, with already-assigned amounts zeroed out
        adj_ledger_amounts, IF(ledger_checks<>0,0,TAKE(ledger_entries,,1)),
        // running sum on the adjusted ledger amounts
        scanner, SCAN(0, adj_ledger_amounts, SUM),
        // drop the processed row
        new_checks, DROP(checks,1),
        // if the scanner is zero, then the ledger check num is already assigned
        // so use it, otherwise if the value from the running sum is
        // less than or equal to the deposit of the check being processed, assign
        // the new check amount, otherwise don't change the array
        new_ledger_checks, IFS(scanner=0,ledger_checks,scanner<=INDEX(checks,1,3),INDEX(checks,1,1),TRUE,ledger_checks),
        // replace the ledger-checks column with the newly calculated array
        new_ledger_entries, HSTACK(DROP(ledger_entries,,-1),new_ledger_checks),
        IF(
            // if new_checks is an error, dropping a row has resulted in an empty array
            // which means the recursion is finished
            ISERROR(AND(new_checks)),
            // and just return the new ledger checks 
            new_ledger_checks,
            // otherwise, process the next row
            RECONCILE(new_checks,new_ledger_entries)
        )
    )

);

Assumptions:

• GL entries for check n are all added to the ledger before check n+1
• Checks table has three columns. Headers aren't important, but column 1 should hold a unique identifier for the check and column 3 should hold the check deposit amount
• The initial call to RECONCILE should have a single-column in the second argument

This assigns entire ledger entries to a single check. If the n+1th ledger entry is too large for what remains in the check's deposit amount, it won't be assigned and will be assigned to the next check. 

Applying a simple SUMIF of the GL Deposit by Check ID next to the check amount would help find rows which aren't fully accounted for. 

=SUMIF(H2#,A2:A3,G2:G9)