Angela Merkel will meet the Greek prime minister, but things aren't looking positive right now. REUTERS/Fabrizio Bensch The Greek government has just over two weeks, until April 8, before it enters a "critical" situation and cash runs dry in Athens.

That's according to Frankfurter Allgemeine Sonntagszeitung, the Sunday edition of the respected German newspaper. The paper says it has seen European Commission estimates that suggest even if Athens taps state-owned enterprises and social funds for money, it will still struggle to make a €465 million ($502.1 million) payment to the International Monetary Fund on April 9.

It's a moving estimate — if Greek Prime Minister Alexis Tsipras or Finance Minister Yanis Varoufakis find some clever way of delaying payments or shuffling funds around, it could be pushed back. But that's not much of a long-term solution. The prospect of running out of funds entirely is now looming.

The government is trying to unlock the extension to its own bailout agreement that was provisionally negotiated in late February. But Athens will not get the €7.2 billion ($7.78 billion) unless it presents a new reform list with more detail on the government's plans and shows some indication the reforms are actually beginning.

Tsipras is in Germany on Monday for his first official visit to Berlin, where he's meeting with Chancellor Angela Merkel.

And the Financial Times has a copy of a letter sent from Tsipras' office to Merkel, showing how strained relations have become. Tsipras says that the current conditions "would make it impossible for any government to service its debt obligations."

Here's another snippet:

I also regret to report that little progress has been made in the negotiations between the technical teams in Brussels and Athens. The reason for the extremely slow progress is that the institutions' technical teams, as well as some of the actors at a higher level, seem to show little regard for the 20th February Eurogroup agreement and are, instead, committed to proceeding along the lines of the Memorandum of Understanding that pre-dates both the 20th February agreement and 25th January 2015 — the date on which the Greek people elected a new government with a mandate to negotiating the new process established by the 20th February Eurogroup agreement. It is difficult to believe that our partners consider that a successful reform drive can be carried out under such restrictive and pressing constraints, including the financial squeeze that my government is currently labouring under.

Tsipras is essentially saying the institutions (formerly known as the troika: the IMF, European Central Bank, and European Commission) are trying to play by the rules of Greece's old bailout, when in fact something entirely new has been agreed — something Angela Merkel does not seem so sure is true.

The back-and-forth of late can't go on much longer if Athens wants to stay solvent. Relations between the two countries are stretched to breaking point, with Greece's politicians pushing for World War II-era reparations and threatening to flood Europe with migrants — while Germany's most popular newspaper calls for "no more billions for the greedy Greeks."

Unless there are more signs of a genuine concrete agreement on Monday, the stakes will rise and Greece will be a day closer to running out of cash — and the potential for an accidental exit from the eurozone.