A free sum could be invested or used for an authorised repayment of an existing residential loan. For the comparison the period, actually available funds and costs must fit both options. An advertised gross investment interest rate and a personal net benefit must not be presented side by side without verification.
Defining a narrow comparison
We consider a single year and €10.000 of free money. For the example it is assumed that an additional loan repayment is permitted and free of charge at the start of the year. With the remaining payment schedule unchanged it would avoid exactly €400 of interest payments over this year. That is a given model assumption, not a statement about your contract.
As an alternative the same amount remains invested. The return used for the comparison is stated after all the costs and taxes assumed here. It is not further reduced by an arbitrarily assumed tax rate.
Presenting the results in euros side by side
With €300 confirmed net return the gap to the assumed interest saving is €100. With €500 net return the investment would be €100 higher in the narrow one‑year result. The arithmetic equality would be €400, i.e. four per cent net on the €10.000 for this year.
This comparison does not claim that such a secure investment would be available. A merely expected value development is not a confirmed net return. If losses are possible or availability is restricted, these characteristics must be assessed separately. They do not disappear because of a higher mean in the table.
Describing the remaining access to money
After a repayment the money used does not simply remain in your account. Whether and how you would obtain credit again later is unresolved. With the investment the access again depends on its actual conditions. Therefore both variants supplement an inventory view: Where are the €10.000 afterwards, what debt exists and which funds would be available for specific other expenditures?
Continuing the calculation with the real conditions
For a real decision you need the permissible repayment conditions, an exact interest effect and the actual net and risk characteristics of the alternative. If the repayment changes the instalment or term, the corresponding payment plan must be used. A longer comparison may also require different calculations.
The example does not provide an individual repayment or investment recommendation. It shows how you reduce an asserted advantage to a comparable, explicitly limited euro calculation and keep personal liquidity visible alongside it.