[sivajyothis7]

Per-component salary amounts on the Salary Structure Assignment, changeable from a date, and read correctly by payroll for each period.

Each component gets its own Currency field on the assignment (HRA, Transport, Other, plus core base, and a read-only Total). On the SSA form each carries a history button and a change button.

Changing a component does not rewrite the submitted assignment. It creates the next one: a new submitted SSA dated from the effective date, carrying every other value forward including payroll_payable_account, without which a Payroll Entry silently drops the employee.

That is what makes payroll correct. hrms picks the assignment for a slip with from_date <= <slip start> ORDER BY from_date DESC and merges every field of that row into the salary-formula namespace, so a slip reads the amounts in force for its own period and regenerating an old slip reproduces its original figures.

Notes on the mechanics:

• Mid-year tax openings (taxable_earnings_till_date, tax_deducted_till_date) are cleared on the new assignment. get_opening_for() only ignores them while from_date < payroll_period.start_date, so carrying them into a later assignment in the same period would re-arm a stale opening on every remaining slip.
• docstatus is reset explicitly after copy_doc: frappe.copy_doc only clears it when local.flags.in_test is false, so tests would otherwise insert straight at docstatus 1 and skip the draft-then-submit path.
• Retro increments stay allowed - dating a change before already-paid periods is normal - but the call reports which periods it reached behind, since no arrears are created automatically.
• A change is refused only when it would be meaningless: an assignment already starts on that date, or a later one already exists.
• wire_salary_structure_formulas() points the Salary Structure earning rows at these fields, dry-run by default. Without it the values are stored and never read. It writes Salary Detail rows directly because amount_based_on_formula is not allow_on_submit and amending the structure would break linked assignments.

The component fields ship as fixtures filtered on module TSI, and sit in their own section after payroll_cost_centers so they do not absorb core's fields.

Not included, by design: the HR Suite Salary Breakup Table band lookup, the override ledger with its Pending/Applied/Superseded states, the daily scheduler, and any write into a submitted assignment.