Ah Shvere Kashyeh

 The Gemoro (.מגילה יד) says קרייתא זו הלילה. The reading of the 

מגילה is considered as the saying of הלל. Therefore we
don’t need to say הלל on Purim.

The מאירי says, that if someone didn't read (hear) the מגילה,
he must say הלל on Purim. (so at least he will be יוצא מצוות הלל)

The Gemoro (18b) says that ר' מאיר didn’t have a מגילה on Purim, 
so he wrote one by heart. 

Shu”t Chasam Sofer O.Ch. 192) R. Yehuda Osad zt"l asked,
"According to the מאירי, ר' מאיר should have said הלל instead
of writing a whole מגילה?"

The חת"ס answered him that saying הלל is only בדיעבד.

My Eidem R. Yoni asked, "The 'קושי doesn't even begin
and the answer of the חת"ס is not to be understood.

All the Gemoro says is, by reading the מגילה you were יוצא the
 מצוות הלל and not that by saying הלל you are יוצא קריאת מגילה"

The Shver iz gebliben mit the Shvere Kashyeh ומצוה ליישב