If there is something on his feet that prevents thewater from reaching them and he is not able to remove it

There are things stuck tomy feet because my feet are very cracked, and
I cannot remove it. Does this invalidate wudoo'? On one occasion
tarmac got stuck to my feet and I removed some of it but I was not
able to removethe rest. Is this prayer permissible?
There is an app for mobile phones which gives the times for prayer and
so on, but it has copyright. Is this permissible?.
Praise be to Allaah.
Firstly:
It is obligatory to remove anything that may prevent water from
reaching the skin, but one is forgiven for a very small amount such as
dirt under the fingernails or in cracks in the feet, after trying hard
to remove it.
Al-Mardaaawi said in al-Insaaf (1/158): If thereis a little dirt under
the fingernails that preventsthe water reaching under them, his
purification is not valid. This was stated by Ibn 'Aqeel. … And it was
said that it is valid, and this isthe correct view.
This is the view to whichthe author (Ibn Qudaamah) was inclined, and
it was the view favoured by Shaykh Taqiy ad-Deen [Ibn Taymiyah]. And
it was said that it is valid on the part of one for whom it is
difficult to avoid it, such as those who do handicrafts or hard work
such as farming and so on.
Shaykh Taqiy ad-Deen said: If there is something of such material
anywhere on the body, such as blood, dough and the like, if it is a
small amount, then itdoes not matter. End quote.
If it is not possible to remove what has got stuck to the skin and
there is a great deal of it, it is not pardoned. In that case you
should wipe over it, as in the case of a plaster cast. Shaykh Ibn
'Uthaymeen (may Allah have mercy on him) said: If there is some
barrier that is preventing the water from reaching the skin in that
case you cannot say that that limb has been washed, but Shaykh
al-Islam (may Allah have mercy on him) said: A small amount is
forgiven, especially for one who has that problem. This applies to
workers who use paint, because they often have one or two spots that
they have either forgotten or couldnot find any means of removing them
at present. According to the opinion of Shaykh al-Islam (may Allah
have mercy on him), they are forgiven for this. But we learn from the
hadeeth –which was narrated by Muslim (243) from 'Umar ibn al-Khattaab
(may Allah be pleased with him), which says that a man did wudoo' and
omitted a place the size of a fingernail on his foot. The Prophet
(blessings and peace of Allah be upon) saw him and said: "Go back and
do your wudoo' properly," so he went back, then he prayed – that
something may not be forgiven even if it is small, if it is possible
to remove it before the time for the prayer ends.In that case he
should remove it, otherwise he should wipe over it and it becomes like
a plaster cast.