Mam sto złotych nadwyżki.

Because Polish usually leaves out subject pronouns when the verb already shows who is doing the action.

Mam is the 1st person singular form of mieć (to have), so it already means I have.
You can say Ja mam..., but that adds emphasis, contrast, or emotion, like I do have...

So the neutral version is simply Mam sto złotych nadwyżki.

After numbers 5 and above, Polish normally uses the counted noun in the genitive plural.

So:

• jeden złoty
• dwa / trzy / cztery złote
• pięć złotych
• sto złotych

That is why sto złotych is correct.

Here sto is the correct form because the numeral is in the basic nominative/accusative form.

After mam, the whole amount phrase works as the object, and with this kind of expression you use sto złotych.

You see stu in other cases, for example:

• nadwyżka stu złotych
• bez stu złotych

So in this sentence, sto is right.

Nadwyżki is genitive singular of nadwyżka.

In this sentence, it works like of surplus or in surplus after the amount expression:

• sto złotych nadwyżki = a hundred zlotys of surplus

This is a common Polish pattern: an amount + a noun in the genitive telling you what that amount refers to.

Compare:

• litr wody = a liter of water
• godzina przerwy = an hour of break
• sto złotych nadwyżki = a hundred zlotys of surplus

No. Here it is singular, not plural.

The basic noun is nadwyżka.
Its genitive singular is nadwyżki.

That can be confusing, because the ending -i often looks plural to English speakers. But in this case it means of surplus, not of surpluses.

Yes. That is also correct.

The two versions are very close in meaning:

• Mam sto złotych nadwyżki
• Mam nadwyżkę stu złotych

The difference is mainly one of structure and emphasis:

• Mam sto złotych nadwyżki highlights the amount
• Mam nadwyżkę stu złotych highlights the surplus itself

The second version can sound a bit more formal or more like written/financial language.

The whole phrase sto złotych nadwyżki functions as the object of mam.

Inside that phrase, different words behave differently:

• sto = numeral
• złotych = genitive plural, because of the numeral
• nadwyżki = genitive singular, meaning of surplus

So even though mam usually takes an accusative object, number expressions in Polish often have this mixed structure inside them.

The normal, neutral order is:

Mam sto złotych nadwyżki.

You can change the order for emphasis, but not every order sounds natural.

For example:

• Sto złotych nadwyżki mam. — possible, but marked/emphatic
• Mam nadwyżki sto złotych. — unnatural

So for a learner, the safest version is the original one.

Yes, it is correct and natural, especially in financial or practical contexts.

That said, in everyday conversation people might also choose other expressions depending on what they mean exactly, for example:

• Mam sto złotych więcej. = I have a hundred zlotys more.
• Jestem sto złotych na plusie. = I’m a hundred zlotys in the black / up by a hundred.
• Zostało mi sto złotych. = I have a hundred zlotys left.

So Mam sto złotych nadwyżki is good Polish, but the most natural alternative depends on the situation.