Praise be to Allah.
If one full year has passed since you acquired the nisaab (minimum threshold) then zakaah must be paid immediately, and it is not permissible to delay it beyond that when one is able to pay. A person does not have the choice of selecting a particular month for paying zakaah on his wealth, rather that month should be after one year has passed, unless he is paying it in advance in Ramadaan, before the year has passed, in which case there is nothing wrong with it. But after the year has passed, it is not permissible to delay it until Ramadaan.
If you pay zakaah on all your wealth, even your monthly salary, this is safer and is the easiest way of paying zakaah on what you have saved of your salary.
We have already explained that riba (usury) is haraam, and it is haraam with regard to building and buying houses. If that loan is riba-based, then you must hasten to repent and get rid of it, seeking that which is with Allaah and fearing His punishment.
With regard to the zakaah of one who is in debt, the correct scholarly view is that the debt cannot be subtracted from the wealth on which zakaah is due. In the example which you mention, you have to pay zakaah on the full one hundred thousand, and you cannot subtract from it the debt that you owe.
And Allaah knows best.