In Islam, the concept of intentions overruling anything else is a recurring theme. If you commit a crime unknowingly (therefore, without the intention of doing so), you are with the mercy of Allah SWT forgiven. If you commit an act with the intention of it being a "good deed" then regardless of the outcome, your ajr is with Allah SWT.
إنَّمَا الأَعْمَالُ بِالنِّياَّتِ وَإنمَّاَ لِكُلِّ امْرِئٍ مَا نَوَى،
فَمَنْ كَانَتْ هِجْرَتُهُ إِلَى اللهِ وَرَسُولِهِ فَهِجْرَتُهُ إِلَى
اللهِ وَرَسُولِهِ، وَمَنْ كَانَتْ هِجْرَتُهُ لِدُنْياَ يُصِيْبُهَا
أَوِ امْرَأَةٍ يَنْكِحُهَا فَهِجْرَتُهُ إِلَى مَا هَاجَرَ إِلَيْه
رَوَاهُ البُخَارِيُّ وَمُسْلِمٌ
And the translation:
It is narrated on the authority of Amirul Mu'minin, Abu Hafs 'Umar bin
al-Khattab, radiyallahu 'anhu, who said: I heard the Messenger of
Allah, sallallahu 'alayhi wasallam, say:
"Actions are (judged) by motives (niyyah), so each man will have what
he intended. Thus, he whose migration (hijrah) was to Allah and His
Messenger, his migration is to Allah and His Messenger; but he whose
migration was for some worldly thing he might gain, or for a wife he
might marry, his migration is to that for which he migrated."
If your intention was indeed to help a poor man for the sake of Allah SWT and for His sake alone, then that is how, inshallah, you will be rewarded; regardless of the true identity of the "pauper" you helped.