Is the questioner considered sinful for spending a sum of money, initially designated for 'Umrah after settling debts, on purchasing new clothes essential for the hijab, with her intention and determination to perform 'Umrah with the first amount she obtains later?

The questioner is not obligated to anything merely by her intention to do something, and this is not considered a vow (nadhr), because a vow is only binding if it is pronounced verbally. Nor has she sinned by spending the aforementioned amount in a way other than what she had intended. However, she must perform the Hajj of Islam and 'Umrah if she has not yet performed them and is capable of doing so.