A securities portfolio is to remain in place while simultaneously securing an additional loan for the purchase of a flat. For your own decision the current portfolio value is therefore not sufficient. You need to know what consequences a loss in value would have under the actually agreed terms and what means you would have for any required adjustment.
Explicitly label the model as fictional
We are not using a real bank contract here and do not assert any usual loan-to-value ratios or automatic margin call obligations. For a real contract, eligible securities, valuation rules, possible changes, response periods, enforcement and liability must be clarified separately by specialists. The following rules are established solely for the calculation example.
Assume a portfolio with a market value of 60.000 euros may in this model cover a loan of up to 50 percent of its then-assessed value. The loan is drawn to the amount of 25.000 euros. At the initial valuation at most 30.000 euros would be covered; the buffer to the utilised amount is 5,000 euros.
Translate the price loss into the coverage rule
If the portfolio value in the example falls by 30 percent, 42.000 euros remain. Of that, 50 percent now only yields 21.000 euros of permissible loan coverage. This is still contrasted with an utilisation of 25.000 euros. The arithmetic shortfall is 4.000 euros.
For the next line we additionally assume that, under the fictional terms, this shortfall must be made up by an appropriate loan repayment. Then the person needs 4.000 euros of free cash. With an external reserve of 2.500 euros outside the portfolio there would remain a gap of 1.500 euros. The loss in value is therefore not just a red number in the portfolio; under the assumed contract it triggers an additional cash requirement.
Do not count the same reserve twice
If the 2.500 euros are at the same time required for already foreseeable debit expenses, they are not available for this adjustment either. Therefore record the concrete competition for funds. Selling further securities can have a different effect on the remaining collateral; it must not be used as an unproblematic substitute without a new coverage calculation.
Require a complete procedure before committing
For a real offer, have it explained according to its own terms what happens in the event of a precisely described price decline and what courses of action are available. Also clarify whether the loan may at all be used for the intended purpose. The appropriate decision follows from the contract, your own capacity to bear the burden and professional advice. The fictitious test provides a concrete follow-up question for that, not a recommendation to buy secured by a portfolio.