Eventually one party will exhaust all their available resources, be that money, gold, desirable trade goods, trust, credibility, etc… and won’t be able to run a deficit anymore.
Lets say you, with your human labor, can use 10 bricks to produce 20 bricks. If you do this every year, your wealth grows. first 10, then 20, then 40, then 80, ect.
In this senario, You can trade with your neighbor and run a 10 brick deficit every year, but you wont exponentially grow your production and wealth. You will have 10 the first year, make 20, trade away 10, then end up where you started. You are sustainable forever, but not growing.
Your house will remain small, and the house of your trading partner will grow ever larger.
Be that quality, quantity, availability, pricing, etc…
Eventually Portugal will exhaust all it’s bricks, and future brick opportunities, that are better in some aspect, relative to French bricks and French future brick opportunities.
And when that happens with every possible thing and opportunity in Portugal, relative to French things and opportunities, then the trade deficit naturally disappears.
That is beside the point of what the long term consequences of running a trade deficit are, if they are sustainable, and how currency exchange impacts these factors.
Humans create new value through labor. It is a renewable resource and you don't run out.